1. Field of the Invention
The present invention relates generally to mechanical vibration reduction and, more particularly, is concerned with a structureborne vibration-compensated motor arrangement having back-to-back twin AC motors.
2. Description of the Prior Art
Numerous potential benefits would stem from the utilization of electric motor propulsion systems for naval surface ships and submarines instead of turbine engine propulsion systems as used up to the present time. AC motors, particularly synchronous types, are simple and reliable.
However, AC motors are generators of variable frequency structureborne vibrations (or noise) which are difficult to damp and attenuate. It is extremely difficult to greatly reduce the structureborne vibrations over a wide frequency range. This basic problem has been a main factor preventing the use of AC propulsion systems for submarines.
The structureborne vibrations caused by AC motor operation result from summation of the electromagnetic forces in conductors and magnetic forces in the magnetic stator core, in space and time. The vibrations appear at the structural interfaces at the outer periphery of the stator core and at the shaft of the AC motor. Sophisticated structural systems which absorb and damp structureborne vibrations over a wide frequency range are required in the stator core support system and at shaft bearings. The vibrations are also transmitted along the propulsion shaft to the ship's propellers, requiring special attenuators to minimize this transmission.
Future naval ships and submarines will require major reductions in structureborne and acoustic vibrations, since detection systems are becoming more sensitive and discriminating. The attenuation and damping of structureborne vibrations is receiving considerable attention. Emphasis is being placed on active vibration damping systems which sense the mechanical forces and apply equal and opposite counter forces at discrete locations on the structures.
However, complete elimination of structureborne vibrations by this approach requires perfect detection and cancellation of the forces at these locations. Consequently, a need exists for a different approach to reducing structureborne vibrations.